1. Field of the Invention
The present invention relates generally to purification devices having photocatalysts. More specifically, the present invention relates to air purification devices having deactivation resistant photocatalysts.
2. Description of the Related Art
Photocatalytic Oxidation (PCO) is a technology used for elimination or reduction of the level of contaminants in a fluid, like air or water, using the chemical action of light. When ultraviolet (UV) light is used to energize the photocatalyst, the technology is more specifically termed Ultraviolet Photocatalytic Oxidation (UV-PCO).
Semiconductors have a sufficiently wide band gap energetic enough to activate water or surface hydroxyls thus creating .OH radicals and electrons have been used in purification systems for elimination of organic contaminants. These materials include, but are not limited to, titanium dioxide (TiO2), zirconium dioxide (ZrO2), zinc oxide (ZnO), calcium titanate (CaTiO3), tin (stannic) dioxide (SnO2), molybdenum trioxide (MoO3), and the like. Of this group, titanium dioxide (TiO2) is among the most widely-used of the semiconductor photocatalysts because of its chemical stability, relatively low cost, and an electronic band gap that is suitable for photoactivation by UV light.
Buildings, vehicles, aircraft, ships and the like may utilize air purification systems to improve the quality of indoor air thus enabling decreased ventilation, create an improved environment, or both. The quality of indoor air is achieved through air purification using either aerosol removal or gaseous contaminant removal technologies. The use of photocatalysis is a proven technology that provides for the removal of gaseous airborne substances such as volatile organic compounds (hereinafter “VOCs”) including toluene and formaldehyde from the air supply.
Photocatalytic air purifiers utilize a substrate or cartridge containing a photocatalyst, usually a titanium oxide based material, that interacts with airborne oxygen and water molecules to form hydroxyl radicals when placed under an appropriate light source, typically an ultraviolet (hereinafter “UV”) light source. The hydroxide radicals attack the contaminants thereby initiating oxidation reactions that convert the contaminants into less harmful compounds, such as water and carbon dioxide.
Titanium dioxide (TiO2), is the most stable oxide form of the transition metal titanium. TiO2 is mostly ionic material composed of Ti+4 cations and O−2 anions. In powder form, TiO2 is white and is widely-used in industry to give whiteness to paint, paper, textiles, inks, plastics, toothpaste, and cosmetics. In crystalline form, TiO2 principally exists as one of three different polymorphic forms: rutile, anatase, and brookite. The two more common polymorphic forms of TiO2, rutile and anatase, have a tetragonal crystal structure, while the less-common brookite form of TiO2 has an orthorhombic crystal structure.
The anatase form of TiO2, which is a low temperature form, has been reported to have the greatest photocatalytic activity of the three polymorphic forms of TiO2 when exposed to UV light. This may be due to a wider optical absorption gap and a smaller electron effective mass in the anatase form that leads to higher mobility of the charge carriers. Anatase is converted to rutile at temperatures above about 600° C. where it is accompanied by crystallite growth and a significant loss of surface area.
The rutile and anatase crystalline structures each have six atoms per unit cell. The anatase form is a body-centered structure and its conventional cell contains two unit cells (i.e., 12 atoms). For both the rutile and anatase forms, titanium atoms are arranged in the crystal structure in such a way that neighboring octahedral units share edges and corners with each other. In the anatase structure, four edges of every octahedral unit are shared edges, as compared within the rutile structure, in which two edges of every octahedral unit are shared edges.
One of the most active of currently-available TiO2 photocatalysts is Degussa Aeroxide TiO2 P25 (Degussa Technical Information TI 1243, Titanium Dioxide P25 as Photocatalyst, March, 2002, Degussa Corporation; Business Line AEROSIL, Parsippany, N.J. 07054) consists of about 80% by weight 20 nm anatase TiO2 crystals and 20% by weight larger, about 40 nm, rutile crystals. On exposure to UV light, electron hole separation can occur. Anatase with a strap gap of 3.20 eV requires higher energy, 385 nm photon, than rutile, 2.95 eV or 420 nm. The hole at the surface takes the form of a hydroxyl radical (.OH) that is a stronger oxidizing agent than ozone or chlorine. The electron on the surface can form active oxygen species through the reduction of dioxygen, perhaps through the formation of superoxide ion, O2− and then by its further reduction to peroxide dianion, O2−2 than can on protonation yield hydrogen peroxide. Hydrogen peroxide is believed to be the principal agent of remote photocatalytic oxidation (PCO), which describes the oxidation of substances that are very close to, but not in direct physical contact with, photoactive TiO2. The presence of both hydroxyl radicals and an active oxygen species are needed for the effective oxidation of formaldehyde to CO2 and H2O over the anatase form of TiO2. P25 crystallites have an average crystallite size of about 20 nm and a BET surface area of about 50 m2/gram. As used herein, BET, stands for the well known method of Brunauer, Emmett, and Teller, (J.A.C.S. 60 (1938) 309) surface science to calculate surface areas of solids by physical adsorption of gas molecules. This has been automated to a certain degree by instruments like the Micromeritics® 2010.
Table 1 provides a comparison of average crystallite size with various measures of surface area, including the anatase and rutile forms of TiO2.
SpecificAverageSurface area/Availablesurfacecrystalliteskeletalsurfacearea,Specificsize,volume,aream2/gsurface areanmm2/cm3m2/cm3anatasem2/g rutile51200800208188610006671741567857571149134875050013011796674441161041060040010494115453649585125003338778134623088072144292867467154002676963163752506559173532356155183332225852193162115549203002005247212861905045222731824743232611744541242501674339252401604238272221483935292071383632311941293430331821213228351711143027371621082825391541032724401501002623
Deactivation of the photocatalyst limits the effectiveness of photocatalytic air purifiers, and can occur reversibly or irreversibly. As the photocatalysts in air purification systems become deactivated, the systems become less efficient. Maintenance is required in order to clean, repair, and replace equipment. This results in increased operating expenses associated with the air purification systems.
Accordingly, there is a need for an air purification system containing a photocatalyst that can resist deactivation in general and/or can resist deactivation due to sudden and/or prolonged rises in contaminant concentration.